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Multi-Frequency Feeds

Objectives

Multi-Frequency feeds are a well known solution to reduce the number of reflectors implemented on a telecommunications satellite. Either it allows decreasing the antenna farm cost by saving (at least) one reflector for a given mission, or it may allow to embark an additional payload on the spacecraft since one reflector is let free. To combine several frequencies within the same feed is easy for scientific, generally narrow bandwidth applications, but very complex for telecommunications ones due to the stringent requirements over a large frequency spectrum.

In this context, the objective of this ARTES 5 study is to develop a Ku/Ka Tx/Rx feed system and to evaluate the achievable performances with a single horn combined with a complex exciter able to separate the following frequency bands, in dual polarizations:

Ku-Tx: 10.95 – 12.75 GHz (dual linear)

Ku-Rx: 13.75 – 14.50 GHz (dual linear)

Ka-Tx: 17.70 – 20.20 GHz (dual circular)

Ka-Rx: 27.50 – 30.00 GHz (dual circular)

First of all, a trade-off has been performed between several concepts. Then, the selected one has been designed, manufactured and tested.

Challenges

The exciter is the first key issue because it turns out that software and computers are not powerful enough to model all RF units chained together and to manage the higher order modes.

So the feed has been separated in two, making an assembly with Ku and Ka-Tx components (removing the Ka-Rx subsystem) and the higher order modes have been kept as low as possible.

The second key issue is the total coupler. This component has been used for years on Earth stations but with a square section main waveguide that implies the necessity of a square to circular taper in order to connect the horn. For Space applications, it increases too much the already important feed total length. New concepts with a circular and an octagonal transversal section have been considered. As a consequence, the coupling factor decreases significantly with respect to the conventional square coupler.

Finally, if a shaped beam is targeted with this type of feed, illuminating a shaped reflector, horn XPD performances shall be very close to the ones obtained with a single Ku bands or Ka bands feed. This is also very challenging because being at the limit of physics. An application in front of a parabolic reflector to create multi-beam coverage with small circular beams is much more realistic.

During the study, and thanks to the help of new RF design software, the performances have been significantly improved compared to the state of the art at the beginning of the study.

Benefits

This feed is the first engineering model of such kind of feed operating signals separation over four wide frequency bands beyond more than one octave (almost 2.5 octave).

Features

The architecture of the system is basically capable to merge within the same exciter a Tx/Rx Ku-band feed and a Tx/Rx Ka-band feed.

The Total Coupler allows the extraction of the upper Ka Rx-band (x), and a waveguide network, including two magic tees and a 3dB hybrid coupler, processes this band into RHCP and LHCP.

The OMJ (x), associated to a waveguide network including two magic tees, couples out the Ku Tx and Rx bands in H and V polarizations. Two diplexers (one for each polarization) separate the Tx band from the Rx one.

The septum polarizer (x) processes the Ka Tx-band into RHCP and LHCP.

Each of the above described functions (and also part of them) can work separately, allowing flexible use of feed subsystems. This Concept is the most innovative for telecoms applications.

Cascade most of these units, at least for all Ku-band parts on the one hand, and all Ka-band parts on the other hand,

Consider the impact of potential higher order modes, very difficult to control over a large bandwidth and with a ratio of almost three between the highest and the lowest frequency,

Design a very large band corrugated horn keeping in mind that for high frequencies, not only the fundamental mode is present within the input throat,

Chain all the above in order to master the S-parameters and radiation pattern of the complete feed.

Such a complex system design, with eight ports and the horn, is to be considered as a very challenging goal.

Plan

The main objectives of the project is to design, manufacture and test a Ku/Ka antenna feed able to operate in full Ku & Ka Tx/Rx bands for Space telecommunication applications.

Current status

The Ku/Ka bands Multi-Frequency Feed has been designed, manufactured and tested.

The RF performances are compliant with the specifications when the Ku-Tx, Ku-Rx and Ka-Tx system is considered, which corresponds to a significant improvement when considering the state of the art in 2005. The corrugated horn RF performances have also been improved thanks to the use of new RF design software developed in parallel to this contract. The horn length has also been significantly reduced by 30%.

When the total coupler subsystem is placed between the horn and the above described exciter in order to extract the Ka-Rx band, the decoupling between ports in the Ka-Rx band is degraded due to impedance mismatching, and consequently cross-polarization is impacted as well. Two main routes have been proposed to overcome these degradations, either by coming back to a square total coupler or to reconsider more standard concept that was assumed to be not challenging enough in ARTES 5 context.

Nevertheless, with the achieved performances, it has been demonstrated that the feed, as it is, could be used for a multi-beam antenna ensuring a mission with User beams in Ku bands and Gateway beams in Ka bands.